Mailing Labels - Parameters

[ValD]

I'm a very new Access user. When I try to create mailing labels using the wizard, I go through all the steps, click 'finish' then up pops a box asking me to enter the 'trim parameter'. If I click 'cancel' print preview of the labels comes up, showing 'Error' all over the page instead of the addresses I want. Can anybody help this IT idiot.